1. Field of the Invention
The present invention relates to power supply systems, and, more particularly to uninterruptible power supply systems that provide highly reliable output power to a load using a slip-ring, wound-rotor type induction machine and flywheel assembly combination and a corresponding method for providing an uninterruptible power supply using the same.
2. Background Art
An uninterruptible power supply (“UPS”) system typically comprises a primary power source, e.g., a utility grid, in combination with one or more means for providing standby alternating current (“AC”) power in the event of a power failure, dips, sags, and the like of the primary power source. Conventional UPS systems typically comprise a primary power source 20 in combination with an engine 50 and generator 60 system and/or with an energy storage means (not shown). See FIG. 4. However, if a power outage, dip or sag of the primary power source 20 occurs with this arrangement—an event that is totally unpredictable—it takes some small, finite amount time to power up and bring the engine 50/generator 60 on line. While the time needed to bring the engine 50/generator 60 on line can be relatively short, for today's computer driven society, a complete power loss for even a fraction of a second could be catastrophic. Furthermore, power dips and sags can cause motors to trip, disrupting manufacturing processes.
An alternative to an engine 50/generator 60 system as a back-up power source includes using an energy storage device, e.g., a battery 40. Batteries 40 store energy, producing direct current (“DC”) and voltage therefrom. Typically, back-up systems that utilize batteries 40 also include (i) a rectifier/converter 100 for rectifying AC current 25 that is provided by the primary power source 20 to DC current 35 and (ii) an inverter 30 for converting DC current 35 and voltage produced by the battery 40 back into an AC current and voltage waveform. See, for example, U.S. Pat. No. 4,634,888 to Deavenport.
Indeed, in practice, batteries 40 produce DC voltage 35 to an inverter 30. The inverter 30 converts DC power into AC power of substantially the same frequency and magnitude. Because batteries 40 enjoy the advantage of an immediate response time, in the event of a power outage, dips or sags of the primary power source 20, a battery-based system can be brought in service instantaneously.
However, there are also several disadvantages to this type of UPS. First, batteries 40 have a finite energy storage capacity. As a result, such a system can provide power only up to the rated storage capacity of the battery 40. If energy demand exceeds the storage capacity of the battery 40, then the demand in excess of the battery storage capacity cannot be met. Accordingly, to provide more energy, a battery 40 with greater energy storage is required. However, battery 40 cost is directly proportional to the stored energy needed. Thus, larger capacity batteries 40 can be prohibitively expensive.
Secondly, batteries 40 still can provide power only for a limited amount of time, which is to say until the battery 40 has drained or, more practically, until the delivered power dips or sags sufficiently to cause motors to trip and the like. Accordingly, yet another power source, e.g., an AC generator and an engine to drive that generator may be required to provide power for longer periods in excess of battery 40 life. Finally, problems can always arise about the storage, maintenance, and life of batteries 40.
An alternative to a battery-based system is an energy storage rotor-, or flywheel, based energy storage system such as those currently being marketed by Beacon Power Corporation of Wilmington, Mass. Flywheel-based energy storage devices comprise relatively simple devices for readily storing and recovering kinetic energy. The flywheel is used in conjunction with an electric motor/generator. Conceptually, as mechanical energy is applied by the rotor of the motor in the flywheel assembly, the flywheel, which is in a tight interference fit with the rotor, spins, storing mechanical kinetic energy.
Most of the kinetic energy of the flywheel assembly is stored in the outermost portion, or rim, of the rotating flywheel. The amount of kinetic energy stored in the flywheel assembly is directly proportional to the inertia and to the square of the rotational velocity of the flywheel. Thus, the thrust of the state of the art is to produce high speed, high capacity flywheels. As the flywheel spins, AC current, and, thus, power, is induced in wires configured and arranged on the stator of the motor/generator for that purpose. However, flywheel assemblies, like batteries, can operate only for a fixed duration, which is to say until the kinetic energy of the flywheel no longer provides sufficient current to power the load.
Therefore, energy storage devices also cannot guarantee fully an uninterruptible power supply. The present invention, however, discloses a UPS system that provides emergency, short-term stored energy using induction-type machines in combination with a flywheel assembly to bridge the gap, i.e., the transition time, between complete failure or short circuit, dips, and/or sags of a primary power source and the start-up and putting on line of a back-up power source.